1. Field of the Invention
Embodiments of the present invention generally relate to a system for use with a well bore tool. Particularly, embodiments of the present invention relate to a force balancing system that decouples the weight of the drill string from the operation and advancement of the tool within a well bore.
2. Description of the Related Art
A percussion method of drilling a well bore into an earthen formation, especially hard rock, involves a cyclic and a spikelike impacting force rather than a steady pressing force imposed by the weight of the drill string. This percussive action produces a superior high rate of penetration versus the traditional drill-by-weight method.
In a percussion drilling application, the drill head needs to be rotated, not for the purpose of delivering energy to break the rock, but rather, for the purpose of positioning the cutting elements mounted on its face to come into contact with fresh rock formations during each subsequent strike. Traditionally, this need is achieved by keying the drill head to the drill string so that the rotation of the drill string, provided by a rotary table mounted on the rig, and in the range of 20 to 40 rpm, is transferred to the drill head.
Usually, percussion drilling tools are pneumatic devices connected to the end of a drill string. Highly compressed air is directed alternately into and out of two separate chambers. One chamber is positioned above a sliding body, commonly known as a piston, and the other chamber is positioned below the sliding body so that the air causes the body to accelerate up and down, reciprocating within the tool housing. During the tool operation, the drill head is kept in contact with the earth at the bottom of a well bore. As the sliding body is directed downward, it forcefully strikes the top of the drill head and causes the rock contacting the drill head to disintegrate.
In a conventional drill-by weight method, the force that is used to press the drill head against the bottom of the formation, commonly called weight-on-bit, is typically between 20,000 to 50,000 pounds. In percussion drilling, since it is the impact force of the reciprocating piston against the drill head that breaks up the formation, this immense weight-on-bit is not needed. However, as the drill head penetrates the formation, it tends to slide out of the housing of the tool. If the drill string is not allowed to descend into the well bore quick enough to keep up with the drill head progression into the formation, the tool can enter into an “opening position” and stop cycling.
On the contrary, however, if the weight of the drill string is not held back properly, the drill string can apply excessive weight onto the drill head. This is also undesirable since the extreme weight-on-bit dramatically increases the frictional torque necessary to rotate the drill head, resulting in excessive damages to related components. The operator thus faces the difficult task of advancing the drill string, on the one hand, quick enough to prevent the tool from opening, and on the other hand, slow enough to avoid pressing the drill head too hard against the formation. This is especially difficult since the operator must hold back most, but not all, of the drill string weight, yet strives to allow gravity to apply just enough force to keep the tool closed. Frictional drag created by contact between the drill string and the walls of the well bore exacerbates this dilemma.
Therefore, there is a need for a methodical system to balance the weight of the drill string and its advancement into the well bore.